The present invention relates to ion lasers and, more particularly, to a gas ion laser having a plurality of series aligned plasma tubes supported within a single resonator structure.
Gas ion lasers produce coherent radiation from an electrical gaseous discharge. The gaseous discharge takes place between a pair of optical reflectors placed in opposition to one another to reflect optical radiation (radiation in the infrared, visible, and/or ultraviolet range of the spectrum) back and forth through a plasma tube assembly having an envelope containing a lasable gaseous medium. The plasma tube is part of an optical cavity extending along the path of the radiation reflected between the optical reflectors (mirrors).
The power output of a laser is dependent, among other things, on the amount of lasable medium in the optical cavity. It has therefore generally been the practice in designing lasers of increased power wattage, i.e., lasers developing more than, say, 5 watts of output power at a selected frequency, to correspondingly increase the size of the plasma tube containing the lasing gas. Typically, operation of such a larger tube results in the generation of sufficient heat in the plasma tube to strain the capabilities of most coolant arrangements. Moreover, the power supply required to operate such a larger plasma tube generally requires a relatively high input power, e.g., 460 volts alternating current.
In view of the above, it has been known to be desirable to divide the volume of the lasing gas between two or more plasma tube assemblies which are axially aligned between a single pair of reflectors, i.e., in a single resonator structure. Radiation generated in the envelope of one tube of such an arrangement passes through the envelope of the other tube as it travels between the optical reflectors. A cascaded tube laser of this type not only can supply an output power level equal to or greater than that which could be expected from each of the individual plasma tubes, it enables differing lasing mediums to be separately enclosed and used in the same laser. Thus, output frequencies characteristic of differing lasing mediums can be obtained from the same laser. Moreover, the cooling requirements for such a laser can be divided between separate cooling systems of the two individual plasma tube assemblies.
While the advantages of such a cascaded tube ion laser have been recognized for some time, there are design problems associated with the same which have prevented their use and implementation except in relatively permanent arrangements. The major problem has to do with the increased length of the light path outside each of the individual plasma tubes which must be provided between the optical mirrors. In this connection, ion lasers include a "resonator structure" designed to hold the optical reflectors rigidly in the orientation relative to one another required to obtain the multiple reflections of the radiation through the lasing medium needed to sustain laser oscillation. The resonator structure also supports the plasma tube assembly appropriately between the mirrors.
Resonator structures are carefully designed to prevent thermal and mechanical stresses from affecting the mirror and tube alignment. U.S. Pat. No. 3,864,029 describes such a resonator structure, including a mounting arrangement for the mirrors.
It will be appreciated that when the length of the optical path is increased, there will be corresponding demands on the tolerances with which the resonator structure must maintain the mirrors in position relative to each other. That is, the increased path length will mean that any slight deviation in the angle of reflection of the radiation from one of the mirrors will be greatly magnified before it reaches the other mirror. However, the increased resonator length which must be provided in order to accommodate more than one plasma tube makes achieving the required tolerances quite difficult. That is, the increased length makes the resonator structure more susceptible to dimensional changes due to thermal or mechanical stresses. In view of this, lasers with cascaded plasma tubes generally have been limited to relatively permanent installations in which a massive base can be included to provide the resistance to thermal and mechanical stresses required to maintain the mirrors and plasma tube assemblies in proper orientation.